Residue in a laboratory vial found last week in a back yard in Northwest Washington has tested positive for the World War I blistering agent mustard, the U.S. Army Corps of Engineers said Wednesday.

The vial was found Aug. 4 during the ongoing excavation at a vacant property at 4825 Glenbrook Rd., in the Spring Valley neighborhood, and the U.S. Army Corps learned of the test results Monday.

The small, dirt-encrusted vial, shaped like half of an hourglass, was about four inches tall. Its seal was partially broken, and it was empty except for the residue, said Daniel Noble, project manager for the engineers' cleanup of the area. "We're talking about a very small quantity," he said.

During World War I, the site of the discovery was a rural tract known as the American University Experiment Station, where the Army tested chemical weapons. The vacant property is now owned by the university.

The U.S. Army Corps, which said air monitors at the site did not detect the presence of any chemical agent, said the vial was removed and posed no danger to the public. It was found about two feet below the surface with other glass refuse.

During the war, some countries packaged liquid mustard into artillery shells that were fired at enemy forces and dispersed the chemical when they exploded, Noble said. It was "a severe blister agent," he said, designed to damage the skin and in particular the respiratory system when inhaled.

Noble said the cleanup at the site was halted after the vial was discovered, and the engineers are reassessing how and when work will continue. He said crews have been working at the property since 2007 and believed they were nearing the end of their efforts. Still, he said, finding the vial wasn't entirely unexpected.

In 2007 at the same site, workers found arsine, another chemical munition that Noble said never entered the arsenal; last year, a vomiting agent, called DA or Clark 1, was found. Experts have been scouring the neighborhood for buried munitions and chemicals since the 1990s.